A significant percentage of the operating time of a helicopter involves low-speed, low-altitude flight regimes and/or hovering operations. Accidents occurring during these modes of helicopter operations involve high vertical descent rates with the helicopter in a near normal flight attitude. While there is some degree of uncertainty vis-a-vis the flight attitude at ground impact of helicopters involved in high-altitude and/or high speed accidents, to the extent that the helicopter pilot is able to exercise the autorotation technique, such helicopters will impact the ground in a near normal flight attitude. In these type of accidents, the landing gear system, whether of the skid-type or the wheel-type, is the first element of the helicopter to impact the ground. As such, landing gear systems are typically designed with the constraint that such systems must be capable of attenuating or dissipating a large degree of the impact energy experienced in a crash landing situation. For example, the FAA requirement for civil aircraft is that such aircraft must exhibit structural integrity after a free fall ground impact from a height of 8.0 inches (equivalent to a sink rate of 6.55 ft/sec). Military aircraft requirements are typically more stringent, requiring structural integrity after a free fall ground impact from a height of 26.8 inches (equivalent to a sink rate of 12 ft/sec). In addition, landing gear systems should be designed so that once the energy-absorbing capability of the landing gear system is exceeded, the landing gear system reaction to the crash landing does not increase the risk of danger to any occupants of the helicopter, e.g., controlled penetration the cockpit and/or cabin areas of the helicopter and/or avoiding rupturing the fuel cells of the helicopter.
The survivability constraint is typically accommodated by design of the landing gear system and/or the undercarriage of the helicopter so that a large percentage of the impact energy arising from a crash landing is attenuated or dissipated by the undercarriage and/or landing gear system. For example, some helicopters are designed with crushable tub structures, i.e., the portion of the fuselage below the passenger compartment, which are designed to crush during a crash landing to attenuate or dissipate the impact energy. This type of design is similar to that used in the automotive industry for attenuating or dissipating the impact energy generated in head-on crashes.
With a skid-type landing gear system, the skids are designed to attenuate the energy generated by normal landings by elastic deformation of the skids. The skids are operative to crush in response to the impact energy of a crash landing. The crushing of metal skids absorbs a significant percentage of the crash landing energy. While skid-type landing gear systems are generally effective, one drawback to such systems is that the degree of degradation of the skids over time due to normal landings may not be readily observable by visual inspection. In addition, replacement of the skids due to degradation arising from normal landings is a labor intensive and expensive process.
Wheel-type landing gear systems typically incorporate a compressible oleo strut subassembly that is operative to attenuate the energy generated by normal landings. Energy attenuation is achieved by stroking which causes compression of a compressible gas in the oleo strut subassembly. This type of energy attenuation is generally effective in decoupling landing loads from the helicopter, and in addition, does not result in any significant degradation of the landing gear system over time due to multiple normal landings. To react the impact energy of a crash landing, wheel-type landing gear systems may employ shear pins which are operative to transfer the impact energy of the crash landing from the oleo strut subassembly to the landing gear trunnion.
The shear pins are inserted in aligned apertures in the oleo strut subassembly and landing gear trunnion and are designed to fail at a predetermined load level (as a result of a crash landing) to effectuate the transfer of the impact energy of the crash landing from the oleo strut subassembly to the landing gear trunnion. There are several disadvantages arising from the use of shear pins. First, shear pins do not have a high degree of durability. Load transfer between the oleo strut subassembly and the landing gear trunnion is subject to a high stress gradient due to the geometry of the shear pins and the corresponding apertures. This can result in local yielding and degradation over time due to multiple normal landings. Secondly, the mechanical degradation of the shear pins and/or aligned apertures is not readily apparent during a visual inspection. In addition, the replacement of worn and/or damaged shear pins and/or the oleo strut subassembly and/or the trunnion (due to aperture wear and/or damage) is a labor intensive, time consuming, and expensive proposition. Finally, shears pins react all of the loads, i.e., vertical, drag, side, and torsional loads, arising from normal landings. It is difficult to analytically predict the degree of damage to the shear pins and corresponding apertures from all loading conditions, and as such, it is difficult to predict with a high degree of certainty at what axial crash load, i.e., ultimate shear loading, the shear pins will shear at. In addition, normal wear and/or degradation of the shear pins and/or corresponding apertures directly affects shear pin tolerances and interfits, which has a significant impact on the ultimate shear loading at which the shear pins fail.
A need exists to develop a durable, predictable, reliable, and maintainable mechanical means to control the functioning of a wheel-type landing gear system in response to a crash landing.